Schoolboy error on installing new brake pads?
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Molto Verboso
150 Sprint Veloce,Vespa GTS 300
Joined: 22 Feb 2010
Posts: 1174
Location: St Neots Cambridgeshire England
Fri Nov 20, 2020 12:27 pm quote
I think I have made an error somehow installing a new set of rear brake pads on my GTS

The pads came today, and while the rear wheel and exhaust were off, I took the chance to work on them.

Firstly I bought these brake pads, because I was happy when I bought these for the front brakes a few years ago, on an older GTS.

However, on fitting them, I noticed that the brake "pistons" were sticking in, so making fitting the fresh brake pads difficult. As the "pistons" were forcing the pads to close in, it was impossible to put the whole assembly back onto the disc!

So, I took the new pads back out, drained the brake fluid out of the brake fluid reservoir at the headset, and opened the bleed valve on the brake assembly to let some fluid out. I then found short piece of wood that allowed me to close the "pistons" and get some clearance so I could put the assembly back onto the disc.

I tightened things up, put the wheel and exhaust back on and noticed that there wasn't any resistance on the brake lever, it was as if there wasn't any fluid back there, and therefore the brake doesn't "brake" anymore!!

Where did I go wrong? and how can I get things sorted out now??

Sheesh, I suppose it means I've got to take off the wheel and exhaust off again!!

S150, Beo 500ie
Joined: 14 Aug 2019
Posts: 437
Location: Bermuda
Fri Nov 20, 2020 12:43 pm quote
Um....are you aware that you need to bleed the brakes after removing fluid and introducing air into the system?
The Hornet (GT200, aka Love Bug) and 'Dimples' - a GTS 300
Joined: 26 Aug 2007
Posts: 39226
Location: Pleasant Hill, CA
Fri Nov 20, 2020 1:26 pm quote
Bleed the rear brake. This can be quite a tiresome task on the GTS if fluid has been removed already, as there are places where air pockets like to stick. Best to reverse bleed.

Next time, before removing the old pads, press the pistons back with a screwdriver or other suitable tool. They should stay where you leave them as long as you don't touch the brake lever. There's no need to remove any fluid.
Molto Verboso
150 Sprint Veloce,Vespa GTS 300
Joined: 22 Feb 2010
Posts: 1174
Location: St Neots Cambridgeshire England
Sat Nov 21, 2020 1:46 am quote
hmmm reverse bleed, I'd never heard of that! How do you go about a reverse bleed Jim?

BV 350
Joined: 22 Nov 2016
Posts: 808
Location: Nebraska
Sat Nov 21, 2020 2:20 am quote
Reverse bleeding forces fluid through the caliper bleeder screw, through the system, back into the master-cylinder reservoir using a large syringe or other device.

Was forced into a similar scenario when my BV350 brakes got air in them. Took me a couple of days to find success, and a quart and a half of fluid. Not doing that again if I can help it.

Or you can make an adaptor for your master cylinder, buy one of the pressure-bleeding sytems, and force fluid through from the master cylinder.

Idea, either way, is to get enough fluid flow to carry the air out of the system, not always possible using the classic bleeding process.

FYI, the proper process when replacing pads is to just press the pistons back in, while keeping an eye on the MC reservoir to assure it does not overflow as fluid is forced back into it. If someone added fluid as the pads wore to maintain fluid level, some may need to be removed. That way, little if any chance of getting air in the system.

If you do not periodically replace your brake fluid, also a good time, after the new pads are in, to run fresh fluid through the system. I siphon out as much old fluid as I can from the MC, fill with fresh fluid, and bleed, using the normal method, until clear, clean fluid comes out the bleeder screw.
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